1. Field of the Invention
The present invention relates generally to a method and apparatus for pyrolytically cracking hydrocarbons. In another aspect, the present invention relates to a method for providing diluent steam for hydrocarbon pyrolysis.
2. Description of the Prior Art
Diluent steam is added to a hydrocarbon pyrolysis feedstock prior to the introduction of the feedstock into the cracking section of a pyrolysis furnace. The presence of diluent steam in the pyrolysis furnace lowers the partial pressure of the hydrocarbon feedstock and improves product yields by promoting higher selectively for the formation of desired olefinic products. One method of diluent steam addition has involved the direct injection of steam into the hydrocarbon feedstock. Another method of diluent steam addition has involved the injection of water into the hydrocarbon feedstock. The water is subsequently vaporized by preheating the water/feedstock mixture in the convection section of the pyrolysis furnace.
In these past methods, the amount of diluent steam addition has been limited by the fuel costs required to generate the diluent steam. The heat required to produce the diluent steam has been provided, for example, by the burning of fuel in a boiler or by the burning of additional fuel in the pyrolysis furnace.
The present invention utilizes waste heat to generate diluent stream for a pyrolysis feedstock. Consequently, the present invention reduces diluent steam generation costs. Further, the present invention allows for the economical use of greater quantities of diluent steam in order to achieve improved product yields.